In a conventional binary, serial data link between a transmitter and a receiver each ‘bit’ of information is encoded by the transmitter as the polarity of the signal. Depending on the nature of the transmitter and the receiver the signal which is transmitted may, for instance, be an electrical signal or optical signal and the signal may be transmitted via any suitable channel, for example via free space or via a suitable guide medium such as a conductive wire or fibre optic cable. The data is transmitted at a fixed rate, with each data bit occupying a period of time equal to the bit period. The receiver is required to determine the polarity of each bit received in order to correctly recover the data sequence. A simple receiver will have a single threshold placed half way between the average ‘1’ level and the average ‘0’ level.
Some digital data streams, especially high-speed serial data streams are sent without an accompanying clock signal. The receiver generates a clock from an approximate frequency reference, and then phase-aligns to the transitions in the data stream with a phase-locked loop (PLL). This process is commonly known as clock and data recovery (CDR).
A phase-locked loop or phase lock loop (PLL) is a control system that tries to generate an output signal whose phase is related to the phase of the input signal. A phase-locked loop circuit compares the phase of the input signal with a phase signal derived from its output oscillator signal and adjusts the frequency of its oscillator to keep the phases matched. The recovered clock determines the times at which the received signal is sampled and quantized.
The common approach to the PLL is to use it to generate a local clock aligned to the transitions in the data. This is shifted by 180 degrees (typically by means of a simple inversion) to provide the clock for sampling the input signal, such that the sampling time is midway between the transition times.
A sampling time midway between the transition times, i.e. in the middle of the bit period, is generally used.